Hysteresis

 The hysteresis of ferromagnetic materials refers to the lag of magnetization or magnetic induction behind the magnetizing field. Thus the irreversible B-H characteristics of ferromagnetic material are known as hysteresis.

   A hysteresis loop `ABCDEA’ is a curve showing the change in magnetic induction of a ferromagnetic material to which an external field is applied as the

Intensity of this field is varied from Hs to Hs   and back again.

Hysteresis loss is as loss of energy is taking a ferromagnetic body through a cycle of magnetization and this loss is represented by the area enclosed by the hysteresis loop.

When the magnetizing field is reduced to zero, the magnetic induction of the material does not come to zero, and this

        Value of magnetic induction is called residual magnetism or retentively .thus the retentively of a specimen when the magnetizing field is reduced from saturation value to zero.
   
Similarly coercive force or coercively of a magnetic specimen is the magnitude of the demagnetizing field required to reduce the residual magnetism to zero. In the residual magnetic induction, coercive field and saturation of the magnetizing field are shown


A study of the hysteresis loop of different magnetic materials helps us to know their magnetic properties .for example let us see the hysteresis curves for soft iron, and steel .from the loop we are able to get the following results.

1.    The area of the loop for steel is greater than that of soft is greater than that of a soft iron, showing a higher energy loss per cycle per m3 for steel.

2.    The B-H curve is steep for soft iron indicating quick magnetization. Further necessary to saturate the soft iron is much smaller than that for steel .further permeability and susceptibility are greater for soft iron.

3.    a greater retentively is appeared in the case   of steel.

4.    The coercivety is greater for steel than for soft iron .it shows that a greater demagnetizing force is required for steel to destroy the residual magnetic induction.

Therefore soft iron is used in electromagnets where a high value of magnetic induction is required. Further since the    hysteresisloss is very small, it is also used in cores of transformers, dynamos and telephone diaphragms. Steel is used in making permanent magnets because it will not easily demagnetized.